1. Field of the Invention
The invention relates to a method for providing a volume of a test substance in a monitoring line which is filled with a test medium and which can be laid along a defined path for determining concentration profiles of liquid and/or gaseous substances. The invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
A method of searching for leaks has been described heretofore in German patent specification 24 31 907. There, a monitoring pipeline is laid along a path to be monitored. The monitoring line is such that liquid or gaseous (i.e., fluid) substances can permeate into its interior. The monitoring line is filled with a test medium, such as for example air. At timed intervals, the column of test medium is pushed through the monitoring line and moved past a probe (sensor) which is sensitive to the substances. Before the column of test medium is conveyed, a defined small volume of a test substance is introduced into the monitoring line as a path marker, the probe likewise being sensitive to the test substance. In this way, a test peak is generated which is used for calibration purposes in terms of both the reference path position within the monitoring tube and the reference peak amplitude of the sensor. Such test peaks can be used to increase the accuracy of detection.
In practice, the monitoring line used is a diffusion tube which is laid along the path to be monitored, for example a pipeline for crude oil or gas. The diffusion tube is filled with air as test medium. When a leak occurs in the path, the substances which have escaped from the path diffuse into the diffusion tube at the location of the leakage and thus form a marker in the column of test medium. Using a suction pump, the column of test medium is moved at timed intervals past gas sensors. In principle, a pressure pump may also be used. The recording of the pumping time against the probe signal permits the location of the leakage in the path to be established.
In order to generate the test peak and thus to calibrate the apparatus, a defined small quantity of hydrogen gas is introduced as test substance into the test medium using a metering apparatus before the test medium is extracted at one end of the path. The metering apparatus may thus be described as a test peak generator. To generate the hydrogen, an electrolysis cell may be provided, which communicates with the monitoring line through a valve. The electrolysis cell and the valve are then controlled, i.e. switched on and off, either electrically by means of cables laid along the path or by radio. Here it is possible to make use of a time control reference, i.e. the generation of a test peak is initiated at specifically set points in time.
The disadvantage of the test peak generator described for the so-called LEOS method is that it is expensive to lay cables, in particular along long paths. LEOS, which is an acronym for as "Leckerkennungs-und Ortungssytem" in German, means leak detection and location system. Controlling the electrolysis cell and the valve by radio is costly as well, because a transmitter and a receiver must be provided.